the term plonker started out as a reference to someone who was forever drunk on cheap wine (cheap wine is nicknamed plonk) this person was usualy a homeless person, or poor person.

today the term plonker is a very light hearted insult. Its not even seen as an insult, its like calling someone a wally. Its in no way ment maliciously. you call someone a plonker when they say or do something stupid.

bob - why didnt my food cook ?
john - you never turned on the oven, you plonker.

Each man takes it in turn to whack the opponents balls with their penis. The receiver must ensure that their penis is held firmly up and out of the firing line so as not to interfear with a clean whack. This is repeated until the lesser man concedes defeat and the dispute is settled.

"Man i think your girlfriends hot; I'll give you a game of plonkers for her"